Composite materials are of great current interest because they provide a very favorable combination of high strength and low density. Typically, a composite material is comprised of fibers of graphite embedded within an epoxy, phenolic or other polymer resin matrix. The more advanced composites which have particularly favorable high strength to density ratio properties are especially attractive for aerospace applications. Typical of other advanced aerospace materials, they present comparative processing difficulties; it is insufficient to make a simple layup of the fibers and resin followed by room temperature curing. Aerospace composite materials not only involve more difficult to fabricate resins but often essentially defect-free finished parts must be produced.
Multicell (multitubular) composite structures have been produced by passing pressure bags through each tube then sealing these pressure bags to each other and to the outer tool or vacuum bag. A vacuum is drawn on the vacuum bag and external autoclave pressure is applied to the pressure bags. However, this method is labor intensive and can be unreliable because of the complexity of the bag end seals. In addition, the bags can apply uneven pressure to the composite prepreg resulting from bridging defects (voids in composite corners caused by pressure deficiencies).
Accordingly, there has been a constant search in this field of art for new methods of making composite tubular structures.